Hob



Aug. 11, 1925. V 4 A. H. CANDEE HOB Filed April 17, 1922' Base Circle 1; m0 NN ATTORNEY.

Patented Aug. llll, 1925..

rillilliifllfl' H. GANDJEE, 01E MILWAUKEE, Wllfii'iifil l'fifilil, ASSIGN'OR TO THE FALK GQEPURA- TIO'N, 015 MILWAUKEE, WISCQNSEN, A C(lfiPORATION WISCONSKN.

HUB.

Application died April 1?, W222. Serial No. 554,067.

all whom, it may 05mm: 7 Be it known thatl, AL AN H. Canons, a citizen of the "United States, residing at Milwaukee,in the county of Milwaukee and 1 State of Wisconsin, have invented a certain new anduseful Improvement in Hobs, of.

I 5 cutting teeth pro ecting fromla cylindrical base andarranged in one cr -more helical series extending thereabout. The teeth are usually formed as. integral "parts of the cylinder, although, insome instances they are removably secured thereto. Integral hobs are ordinarily produced by providing a cylinder having one or more integral threads on the surfacethereof and by interrupting the thread or threads at regular.

intervals by transverse gashesor flutes to form one or more helical, series of separate projections constituting the res ective teeth. The face at theleading en of each projection thus formed contains the cutting edges of each .tooth' andwill lfiereinafter be referred to as the cutting ace.-

The sides and ,top of each projection are tapered oil rearwardly from the cutting face to secure proper ,cuttin clearance. ,This is ordinarily accomplishe in a relieving lathe or grinder in which the tool or grinding element is advanced parallel to the axis of the hobblank, during the rotationofthe 40 blank, so as to iollow the lead of the hob thread, the actual relief being effected by an intermittent shifting of the tool or grind- -ingelcment during such advance.

The,

sharpened from time to time by grinding the cutting faces of the respective teeth.

' Since the original cutting faces are thus destroyed and the-fresh cutting edges occur at new. positions along the relieved sides of 5c the teeth,,it isobvious that the shape of the relieved sides determines the characteristics of the .fresh cutting edges.

4 Throughout the following description, the

' term thread will be employed to designate osfthat imaginary helical thread defined by the I bodies of the finished hob is' sharpened and recutting faces of the successive teeth and whose sides constitute imaginary'helicoidal surfaces containing the side cutting edges or the successive teeth. The lead Wlll be used to designate the length along the hob axis of a thread or helix for one revolution. The term pressure angle will also be employed in its'usual sense with reference to gear teeth and worm threads, the term eilective pressure angle being employed to desi ate the theoretical pressure angle of the imaginary helicoidal surface constituting a side of the hob thread.

Hobs are either single or multiple threaded, the number, shape and lead of the threads being dependent upon the form of tooth which the hob is designed to generate. The gashes or flutes extend substantially normal to the threads, so that in those hobs in which thelead of the threads is long, the gashes extend spirally, although in some instances in which the lead is very short the ga shes often extend parallel to the hob axis. Heb threads are known to be either straight or curved sided, dependent upon whether the'intersection of an axial'plane, or a plane normal to a thread, with the side of the thread, is a stralght or a curved line.

.'The hob was originally considered to be 7 a rack wrapped about a cylinder and, as such, it was believed that, to generate involute gears, a hob thread should be straight sided. It is now generally recognized that straight sided hob threads will not generate theoretically correct involute gears, various attempts having been made to produce hobs having threads of correct form. Although these attempts have not been entirely successful, some have been based upon the more recent correct conception that a hob is es-' sentially ahelical gear andthat, to generate involute gears, the hob thread must be of involute form.

The attemptsheretofore made upon this recent conception as a working basis have involute surfaces upon the actual sides of the hob teeth, without regard to the result ing shapeof the effective sides of the thread defined by the cutting edges of successive teeth.- Actually, little attention has been paid to the fact that the relieved sides of the teeth do not coincide with the imaginary surfaces constituting the effective sides of the thread. Furthermore, so far as I am aware, no attention whatever has been given resharpening.

to the fact that the relieved sides :of the teeth must be of uniform lead at all; diameters of the hob in order to retain the characteristic form of the cutting edges after The present invention is predicated upon- I the conception that a hob for generating involute gears must be essentially an involute helical gear, and that, as such, the effective sides of the thread, as distinguished from the actual relieved sides of a tooth,

- must be of involute form. Properly considered, the form of the efiective side of a .hob thread is defined by the cutting edges of It is the general aim of the present invention to so shape the relieved sides of the teeth of a hob that the resulting cutting edges of successive teeth will define imagi nary helicoidal surfaces of vinvdlute form constituting the effective sides of the hob thread.

Another object is the provision of a hob having these characteristics and in which the sides of the teeth are .so formed that the cutting edges, and consequently the efr fective sides of the threads, will retain their characteristic form after resharpenin For purpose of explanation, I shal select a hob in which the cutting sides of the gashes or flutes are generated by. a radius of the hob and shall describe one method by which the sides of the teeth of sucha hob may be formed to obtain the desired characteristics.

Other objects and advantages will hereinafter appear.

In the accompanying drawings:

Figure ,1 is a plan view diagrammatically illustrating a partially completed hob produced in accordance with the teachings of the present invention.

Figure 2 isa fragmentary sectional view, on a somewhat larger scale, taken substantially along the line 2-2 of Figure 1.

Figure 3 is a fragmentary sectional view taken substantially on the line 3-3 of Figures 1 and 2.

Figure 4:, is a view similar to that of Fig ure 3 showing the contour of aside of a relieved tooth.

The hob selected for illustration com rises a cylindrical base 10 having a double tiread 11 extending thereabout. These threads are interrupted in the .usual mannerlby spiral flutes or gashes 1 2 to form the bodies 130i the respective teeth. The top faces 14 of the teeth are relieved in the usual manner, the side faces 15 being'axially relieved by the'method fully described in my copending application, Serial No. 547,031, filed March 27, 1922. By the method therein disclosed, the side faces of the teeth constitute helicoidal surfaces of'uniform lead at all diameters of the hob.

As above pointed out, the hob is considered as an involute helical gear and the teeth to be generated thereby are considered as the teeth of a conjugate gear. -Upon this hypothesis the sides of a' hob thread, which corresponds to the face of the tooth of a helical gear, must be of such form that an involute of a circle is obtained at its intersection with a plane of rotation of the hob. The diameter of thebase circle for this involute, as well as the pitch circle, pressure angle, lead and other dimensions of the hob, can be determined by the usual principles applicable to spiral gears.

In the drawings, the line XX indicates the axis of the hob; the line 2--2 a through the hob normal to the axis and referred to as a plane of rotation; the point A, the intersection of the axis of the hob with the plane of rotation the point 0, the origin ofthe involute curve, and the line 3-3, an axial plane of the hob passing through point 0. Thus, in Figure 2, the intersection of the side of a thread with the plane of rotation is represented by the involute curve Op, derived from the base circle whose radius is AO. Thiscurve may obviously be expressed in polar-coordinates plane by the followingcharacteristic polar equation V Y r o r 0=tan arc cos -arc cos (1) in which 1- is the radius' AO of the base circle; .9 is the radius vector or the distance of any point p from the center A; and 0 is the angle OAp in radians between the radius vector and the polar axis AO.

The curve OP of Figure 3 re resents the intersection of the same side 0 the thread with an axial plane of the hob. This curve may be expressed in rectangularcoordinates with reference to the axis of the hob and a given diameter of the hob as base lines. For this purpose, the line XX will be considered as the X-axis and the line AO ofthe -axis.

Let point'P (Fig. 3) represent any point on the curve OP whose known distance from the hob axis is represented by the distance y. To determine its distance x from the line AO select a point p on the curve 0 whose distance Ap equal to y.

By construction, both points P and p lie from the hob axis s masher in the side of the thread and both are the same distance from the hob axis. Since the lead of the side of the thread is uniform at.

all diameters, it is clear that the distance at of the point P from the selected plane of rotation, is proportional to the angle 6 between the point p and the selected plane of L in which L is the lead of the hob thread. Substituting in equation (2) the value of 0 as expressed in equation (1) and also substituting for 8 its equivalent y, we have.

L r r 01:- Zr (tan arc cos 5f arc cos (3) which constitutes the? equation in rectangular coordinates of the curve OP. It is obvious that by giving y several values in-this-equation and solving for w in each instance the rectangular coordinates of several points of the curve OP may be determined and the curve plotted.

It now remains to determine a method by which the actual sides of the hob teeth may be given such form that the resulting cutting edges of successive teeth will'line in application above identified, so that the sides I of the teeth thus relieved constitute heli- .coidal surfaces of uniform lead at all diameters of the hob. The tool is ordinarily clamped in the lathe in such position that itsgcutting edge lies in an axial plane of the In the case of straight fluted hobs in which the cutting face of each of the hob teeth lies in an axial plane of the hob, it is obvious that the side cutting edges of the teeth are represented by the curve UP and that under these conditions the curve OP also represents the correct form of the cutting edge of the lathe tool.

In the case of s irally fluted hobs, however, the cutting edges of the hob teeth do not lie inan axial plane of the hob and do not at any time during the relievingprocess ,coincide with the edge of the relieving tool,

so that a tool edge of somewhat modified form is required in the production o'f'hobs of this type. This matter'is fullytreated in my copending application, Serial No. 544,424, filed Marchsl7, 1922, in which a formula is iven by which the required correction in te angular setting of the edge of the lathe tool may be ascertained. This. formula is applicable to hobs having straight sided threads. A formula of somewhat modified form .is necessary in the case of curve sided threads, by which the correct modified form of tool cutting 'edge may be ascertained. This modified formula is as follows:

in which G is the lead of the gash helix; L is the lead of the thread; L is the lead of the actual. relieved side of the hob tooth; w is the w-coordinate of any given point P. on the curve UP; and mis the w-coordinate of the point p on the modified curve whose y-coordinate is the same as that of P.

It is clear that by the use of this formula in connection with formula (3) the rectangular coordinates of the several points on curve @112, may be computed and the curve plotted. This curve, as shown in Figure 4, represents the intersection of an axial plane of the hob 5 with the actual side'of a relieved tooth andconsequently represents the correct form for the edge of'thetool necessary to produce a hob having the desired characteristics.

The formuladast given is applicable to 100 spirally fluted hobs in which the cutting faces of the teeth are helicoidal surfaces generated by a radius of the hob. In the case of hobs having hooked teeth, this formula will have to be somewhat modified.

ll claim:

1. A spirally gashed hob for generating involute gears having one or more helical series of cutting teeth so relieved that the sides of the imaginary thread defined by the no comprising one or more helical series of cutting teeth having relieved sides constituting helicoidal surfaces of uniform lead at all diameters of the hob and'of such form 2 that the resultant cutting edges of successive teeth define imaginary helicoidal surfaces ofinvolute form.

4. A hob for generatin involute gears comprising one or more elical'series of &

cutting teeth having relieved sides consti-- tuting helicoidal surfaces of uniform lead at all diameters of the hob and of such form that the resultant side cutting edges of suc- 5- cessive teeth lie in imaginary helicoidal sur-' faces whose intersections with a lane normal to the axis of the hob constltute involntesof a. circle. 7

5.'A hob for generating involute gears 1 comprising one or more helical series of Cutting teeth having relievedsides s0 shaped In witness whereof, I hereunto subscribe my name this 7th day of April, 1922.

ALLAN H. CANDEE. 

